hCB-CD34⁺ cells are transduced with remarkably high efficiency and maintain their progenitor cell phenotype after lentiviral magselectofection. hCB-CD34⁺ cells were transduced at a low cell density of 1.5 × 10⁵ cells/mL without cell stimulation before magselectofection. (A) Fluorescence (490/509 nm) and bright-field microscopy images of the hCB-CD34⁺ cells taken on day 3 after magselectofection with the magnetic complexes at 2 fg Fe/VP. Bar represents 100 μm. Objective Achroplan 20×/0.4 NA. (B) Histogram plots of the untreated hCB-CD34⁺ cells (Untx), cells transduced using the standard infection protocol or viral magselectofection with the complexes formulated at 2 or 20 fg Fe/VP. (C) GFP⁺ BFU-E colony at 14 days after magselectofection. Objective 5×/0.12. (D-E) Quantification of BFU-E and CFU-GM colonies at 14 days after magselectofection (N = 1-3). (F) Percentage of cells released from the column (N = 3-5) for CD34⁺ cells alone, with addition of lentiviral vector (LVP) and magselectofection (2 fg Fe/LVP).